To better understand the influence of fractures on oil and gas production, a core analysis and a detailed logging program are usually required. The general objectives of such a program are first to identify fractures, second to orient the fractures, and third to predict their influence on the production of individual wells. To accomplish this detection and characterization of fractures in reservoirs, two logging measurement techniques are used to image the subsurface surrounding the borehole. These are the formation micro scanner (FMS) and the borehole televiewer (BHTV). Existing commercial BHTV and FMS techniques do not give a quantitative measure of fracture aperture. Fracture orientation is readily obtained for inclined fractures with either BHTV or FMS logs, but the orientation of vertical fractures is commonly ambiguous on both logs.
Fortunately, crossed dipole acoustic logging can provide detailed information on the anisotropy of the subsurface formation. This method, based on the detection of split flexural modes, was developed by Schlumberger and designed to determine the orientation of vertical fractures and microcracks, as well as differences in horizontal stresses caused by azimuthal anisotropy. In fact, the present dipole-shear anisotropy technique has been used to determine the maximum stress direction of hydraulic fractures and to detect fracture zones behind cased wells for perforation decisions.
The present methodology used for processing borehole dipole sonic logs is based on the transversely isotropic Green's function defined by having the axis of symmetry perpendicular to the axis of the borehole and by five stiffness constants (i.e. C11, C13, C33, C44, and C66) to characterize the formation. As a consequence, the data recorded by the cross-dipole acoustic system is processed for the azimuthal anisotropy of the formation only. The data is not processed for fracture aperture, fracture density, fluid properties, fracture separation, and fractured zones not intersected by the well. An approach is needed to model these parameters. In particular, since large fractures account for most of fluid flow, and wells may intersect only a few of these fractures, a technique is needed to detect those fractures near the well. Similar techniques can be applied to detect new fractures that may be developed near the well after hydraulic fracturing.